The present invention relates to a ferroelectric liquid crystal display element which realizes analog-type gray scale display, and a method for producing the same.
Ferroelectric liquid crystal was first synthesized by R. B. Meyer in 1975. Thereafter, it was confirmed by N. A. Clark and S. T. Lagerwall that in the case where a gap between substrates provided vis-a-vis is narrowed, ferroelectric liquid crystal exhibits bistability.
Further, a variety of researches have been done on ferroelectric liquid crystal to seek for application of its utility, and consequently various display modes have been contrived. A typical one among them is a display mode in which liquid crystal material has positive dielectric anisotropy and is aligned in a C1 orientation. The C1 orientation refers to an alignment wherein liquid crystal molecules 22 are tilted at wider angles with respect to smectic layers 21 which are bent to form a chevron structure, as shown in FIG. 2(a).
A mode generally called xcfx84-Vmin mode, in which liquid crystal material has negative dielectric anisotropy and is aligned in a C2 orientation, is also typical. The C2 orientation refers to an orientation, as shown in FIG. 2(b), wherein the liquid crystal molecules 22 are tilted at narrower angles with respect to the smectic layers 21 which are bent to form the chevron structure. This mode is characterized in that due to the C2 orientation, high response is easily realized as compared with the case of the C1 orientation, and that since the liquid crystal material has negative dielectric anisotropy, a memory angle is increased by a bias voltage.
In the foregoing two modes, since the liquid crystal molecules 22 basically exhibit binary bistability, only gray scale display with two gray levels (black and white) is obtained in display in the case where either of the modes is applied to a display device. Therefore, as to such a conventional display device, to increase the number of gray levels in gray scale was an object to be achieved. By using the frame division driving scheme, the pixel division scheme, or the like, the number of gray levels in gray scale can be increased to some extent, but to further increase the number of gray levels in gray scale is hardly possible because of limitations of the liquid crystal material and the device structure. Particularly to achieve the gray scale of 256 gray levels per one color of RGB which is required for full color display is extremely impossible.
Therefore, realization of the analog-type gray scale display in a ferroelectric liquid crystal display element has been demanded. To meet this demand, the following, for example, have conventionally been proposed: a method wherein analog-type gray scale display is realized by generating domains using as nuclei fine particles mixed in liquid crystal (see the Japanese Publication for Laid-Open Patent Application No. 194693/1994 (Tokukaihei 6-194693), date of publication: Jul. 15, 1994); and a method wherein analog-type gray scale display is realized by generating domains using polymers dispersed in liquid crystal (see the Japanese Publication for Laid-Open Patent Application No. 236830/1997 (Tokukaihei 9-236830), date of publication: Sep. 9, 1997, which corresponds to the U.S. patent application Ser. No. 09/066215).
In the case where these methods are applied to analog-type gray scale display, however, there arises a problem of display history that display of a current frame is affected by a state of a display of a previous frame. More specifically, in the case where signals of black data are continuously applied to pixels and thereafter signals of half tones are continuously applied to pixels, the intensity of light transmitted through pixels, supposing that display history would not exist, immediately changes from a black level to a half-tone level as shown in FIG. 15(a). In contrast, since display history exists in practice, as shown in FIG. 15(b), the intensity of the transmitted light does not immediately change to a half-tone level after application of half-tone data and gradually changes to reach the half-tone level after several frames.
Conventionally, such a scheme as using a conductive alignment film or shortening each frame time of a driving signal (The 22nd Japanese Liquid Crystal Conference, pp.191-192) has been preferably applied to solve the foregoing problem. There has been also proposed a scheme of giving conductivity to an alignment film by mixing a conductive substance therein (Japanese Journal of Applied Physics, Vol. 28, No. 1 January 1989, pp. L116-L118, date of publication: Dec. 15, 1988). This scheme, however, is used for enhancing the bistability of liquid crystal.
Incidentally, the following description will explain behaviors of ferroelectric liquid crystal in the case where an ionic substance is mixed in the ferroelectric liquid crystal.
Usually, mixture of an ionic substance in ferroelectric liquid crystal adversely affects characteristics of ferroelectric liquid crystal. For example, a burn-in phenomenon occurring to a display screen, and a uni-stable state, i.e., a state in which only one state of bistable states is stabilized, are induced. This is caused by accumulation of ions in liquid crystal. As a method for preventing such adverse influences of ions, there have been proposed a method of causing an alignment film to attract ionic substances (see the Japanese Publication for Laid-Open Patent Application No. 245962/1996 (Tokukaihei 8-245962), date of publication: Sep. 24, 1996) and a method of injecting ion-removed liquid crystal into an empty cell (see the Japanese Publication for Laid-Open Patent Application No. 181508/1995 (Tokukaihei 7-181508), date of publication: Jul. 21, 1995).
In some cases, ions enhance characteristics of ferroelectric liquid crystal. For example, in the case where ferroelectric liquid crystal is driven by TFTs (thin film transistors), mixture of ions produces an electrical double layer in the ferroelectric liquid crystal, which, without raising a signal voltage, prevents a voltage applied to the liquid crystal from lowering (see the Japanese Publication for Laid-Open Patent Application No. 43643/1997 (Tokukaihei 9-43643), date of publication: Feb. 14, 1997). In this method, liquid crystal is injected into a cell after ionic substances are directly mixed in the liquid crystal.
Actually, however, the problem of display history cannot be sufficiently solved by using a conductive alignment film or by shortening a frame time of a driving signal. Besides, by the method of mixing ions into liquid crystal before injection of the liquid crystal, ions are not homogeneously dispersed in the liquid crystal due to a chromatography phenomenon, whereby domains also do not become homogeneous. Therefore, this method cannot be effective to solve the problem of display history.
The object of the present invention is to provide a liquid crystal display element which is capable of forming fine domains in liquid crystal and drastically reducing influences of display history, as well as a method for producing the same.
To achieve the foregoing object, a liquid crystal display element of the present invention, which includes a pair of substrates with an insulating property on each of which electrodes are formed, an alignment film provided on at least one of the substrates so as to cover the electrodes thereon, and a liquid crystal layer made of a ferroelectric liquid crystal material sealed between the substrates, is characterized in that the alignment film contains a substance with ionic bond in the vicinity of the surface or inside thereof.
According to the foregoing arrangement, the alignment film contains the substance with ionic bond in the vicinity of its surface or inside. Therefore, ions are eluted from the alignment film into the liquid crystal layer, whereby ions are homogeneously dispersed in the liquid crystal layer. Therefore, during the half-tone display, fine domains are formed in the liquid crystal layer due to such ions, while influences of display history of a previous period are drastically reduced. In result, gray scale display with not less than two gray levels can be stably conducted.
Incidentally, cases meant by the description xe2x80x9cthe alignment film contains a substance with ionic bond in the vicinity of its surface or insidexe2x80x9d include a case where the substance with ionic bond adheres to a surface of the alignment film.
A method of the present invention for producing a liquid crystal display element having a pair of substrates with an insulating property on each of which electrodes are formed, an alignment film provided on at least one of the substrates so as to cover the electrodes, and a liquid crystal layer made of a ferroelectric liquid crystal material sealed between the substrates is characterized by having the steps of (i) mixing a substance with ionic bond in an alignment film material for forming the alignment film, (ii) applying the alignment film material to at least one of the paired substrates so as to form the alignment film, and (iii) bonding the substrates to each other after the mixing and applying steps, and injecting the ferroelectric liquid crystal material between the bonded substrates.
By the foregoing method, since a substance with ionic bond is previously mixed in an alignment film material, the alignment film formed by application of the alignment film material to the substrates contains the substance with ionic bond. Therefore, this method enables production of a liquid crystal display element provided with an alignment film containing the substance with ionic bond, without complicated steps. Thus, it is possible to easily provide a liquid crystal display element whose gray scale display characteristics are remarkably improved.
Another method of the present invention for producing a liquid crystal element having a pair of substrates with an insulating property on each of which electrodes are formed, an alignment film provided at least on one of the substrates so as to cover the electrodes, and a liquid crystal layer made of a ferroelectric liquid crystal material sealed between the substrates is characterized by comprising the steps of (i) either soaking the substrates into a solution of a substance with ionic bond or applying the solution to the substrates, either before formation of the alignment film, after formation of the alignment film, or after application of an aligning operation with respect to the alignment film already formed, and (ii) bonding the substrates to each other after the forgoing step, and injecting the ferroelectric liquid crystal material between the bonded substrates.
By the foregoing method, since the substrates are soaked into a solution of a substance with ionic bond or the solution is applied to the substrates at any one of the foregoing stages, the substance with ionic bond is infiltrated into the alignment film already formed. This ensures that in the case where the substance with ionic bond is not contained in an alignment film material, the solution of the substance is applied to the alignment film through any step other than the alignment film formation step, resulting in that the substance with ionic bond is infiltrated into the formed alignment film. Therefore, by the method, without complicated steps, a liquid crystal display element with an alignment film which contains a substance with ionic bond in the vicinity of the surface or inside thereof can be produced simply by either soaking the substrates into the solution or applying the solution to the substrates at a desired stage. As a result, it is possible to provide a liquid crystal display device whose gray scale display characteristics are remarkably improved.
Furthermore, still another method of the present invention for producing a liquid crystal display element having a pair of substrates with an insulating property on each of which electrodes are formed, an alignment film provided on at least one of the substrates so as to cover the electrodes, and a liquid crystal layer made of a ferroelectric liquid crystal material sealed between the substrates is characterized by comprising the steps of (i) forming the alignment film on at least one of the paired substrates, and (ii) infiltrating a substance with ionic bond into the alignment film.
By the foregoing method, since a substance with ionic bond is infiltrated into an alignment film already formed, even if the substance with ionic bond is not previously contained in the alignment film, it is possible to produce a liquid crystal display element with an alignment film which contains a substance with ionic bond in the vicinity of the surface or inside thereof. Therefore, by the method, without complicated steps, a liquid crystal display element whose gray scale display characteristics are remarkably improved can be easily produced.
Furthermore, still another method of the present invention for producing a liquid crystal display element having a pair of substrates with an insulating property on each of which electrodes are formed, an alignment film provided on at least one of the substrates so as to cover the electrodes, and a liquid crystal layer made of a ferroelectric liquid crystal material sealed between the substrates is characterized by comprising the steps of (i) infiltrating a substance with ionic bond into the substrates and (ii) forming an alignment film on at least one of the paired substrates.
By the foregoing method, since an alignment film is formed on the substrates into which a substance with ionic bond is infiltrated, it is possible to produce a liquid crystal display element with an alignment film which contains a substance with ionic bond in the vicinity of the surface or inside thereof, if the substance with ionic bond is not previously contained in the alignment film. Therefore, by the method, without complicated steps, a liquid crystal display element whose gray scale display characteristics are remarkably improved can be easily produced.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.